Protection of circuit components against high-energy transients, such as due to nuclear explosions or lightning, requires that the protection circuitry divert the transients or attenuate them so that the breakdown level of the circuit components is not attained. For electrical lines carrying power to the circuit components, devices such as varistors and spark gaps have been employed in order to divert the transient to ground before it reaches the circuit components.
A varistor is a resistor whose current/voltage characteristic is very non-linear, so that its resistance varies substantially with voltage. If the voltage across it exceeds a predetermined value (slightly above the normal voltage to ground of the power transmission line), the high voltage of the transient results in the varistor exhibiting a very low impedance. Thus, a varistor connected between an electrical line and ground can effectively discharge the high-energy transient to ground.
Commercially available varistors, such as metal oxide varistors, are typically constructed in a manner that limits their suitability for protecting circuit components of a weapons system against damage from transients caused by the high energy, high frequency, and high thermal stress that can be associated with, for example, a nuclear explosion.
In typical varistor assemblies, a varistor is connected to an electrical line by a single contact or terminal. Unfortunately, for high frequency transients on the order of, for example, 2-3 MHz, a significant amount of self-inductance exists in a copper strip that connects the single input/output terminal to the varistor. As a result of this inductance, a substantial amount of the transient will reach the circuit components and will not be diverted to ground, e.g., on the order of 8 kilovolts will not be diverted for a 10 kilovolt 100 MHz transient.
Also, such varistor assemblies fail in an open-circuit mode for very high energy transients. The occurrence of such high-energy transients is regarded as so rare that there will be time to replace the protection circuitry. The failure of the varistor in the open-circuit mode is regarded as acceptable as long as circuit components are protected from the initial high-energy transient. In a weapons system, however, an open-circuit failure mode is not acceptable. In a nuclear weapons scenario, very high-energy transients are to be expected. Furthermore, the weapons system typically will have back-up power for driving circuit components. Therefore, the weapons system can still operate after a very high-energy transient, but is disabled by any subsequent transients because of the open-circuit condition of the failed varistor.
Finally, thermal stress is created as a result of changes in temperature when there is pressure contact between the varistor, typically ceramic, and a single metal plate or strip. The expansion and contraction of the metal plate creates stress on the varistor, which is a significant problem because a prime failure mode for the varistor occurs when the varistor is cracked.
A spark gap is a pair of electrodes so designed that a spark or an arc can safely pass between them when the voltage across them exceeds a predetermined breakdown value. When arcing occurs, the gap region is ionized and electrons emitted by the cathode cross the gap and reach the anode. A spark gap between the conductors in a DC power line protects against damage to circuit components due to high-energy transients. Unfortunately, although an arc will begin to pass between the electrodes only when the breakdown voltage is exceeded, the normal system voltage to the transient will usually be sufficient to maintain the arc across the gap, necessitating the circuit being momentarily switched off.
Several methods have been employed in order to terminate the arc when the voltage on the power line returns to normal. Horn gaps extend the length of the arc so that it extinguishes more easily by using extended electrodes along which the extremely hot arc will travel. Magnetic fields can be applied to the gap to slow down or divert the electrons crossing the gap. Additionally, the gap can be cooled so that the arc will terminate.